Conversations that are carried on over telephonic equipment are particularly vulnerable to eavesdropping using any of the many ways known to intercept voice audio from telephone conversations. One approach is to penetrate the telephone circuit between the telephone instrument and the telephone company switching office. A wide variety of eavesdropping devices are known that can be connected to the telephone circuit for monitoring telephone conversations. For many years efforts have been made to maintain the confidentiality of conversations between parties over such telecommunications equipment.
Telephone voice encryption devices have been devised to address the problem noted above by digitally encoding voice signals before transmission and decoding the voice signals at the receiving end. For example, systems have been devised that utilize high-grade algorithms, such as U.S. Data Encryption Standard (DES) and proprietary algorithms, preferably in conjunction with RSA Public Key Technology (RSA Data Security, Inc., Redwood City, Calif.), such as the Motorola, Inc., Government Electronics Group (Scottsdale, Ariz.) SECTEL series, or unique systems such as the U.S. government STU-III.
Many schemes exist for providing a secure communication channel while a telephone is in use (e.g. U.S. Pat. Nos. 3,718,765, 4,920,567, 5,181,243, 5,805,635, 5,911,119, 5,963,621). In most cases, commercial telephone encryption equipment cannot provide total voice security due to the eventual breakdown of intercepted data. Nonetheless, encryption remains the most popular technique for maintaining secrecy when the conversational content is sensitive in nature.
Although the foregoing prior art techniques have proven useful in providing secure voice communications over an active telecommunications channel, it is nonetheless still possible to eavesdrop on conversations in a room containing telecommunications equipment (e.g. telephone, facsimile machine, etc.) when the equipment is idle (i.e. on-hook). Unless one takes extraordinary precautions, virtually any equipment connected to a telephone line generates a microphonic signal. This is primarily due to the fact that most wired telecommunications systems use transformers to transmit duplex information over a pair of wires. With the advent of better and more sophisticated amplification and signal processing equipment, signals in the sub-micro-volt range can easily be monitored. This gives rise to a serious security risk since it is possible for an eavesdropper to connect listening equipment on the two conductors leading to the room in order to monitor conversations in the room, even while the telephone or facsimile machine, etc. is on-hook.
In older analogue equipment, mechanical switches have been used to physically disconnect the telecommunications equipment, as set forth in U.S. Pat. Nos. 4,675,901 and 4,949,375. However, in digital transmission systems the use of switches becomes problematic since considerable non-voice information must constantly be sent to and received from the telecommunications device. In effect, the device is virtually always on-line although there may be no voice, video, facsimile or such data being exchanged. Many of these digital transmission systems have encryption schemes, as noted above, which make voluntary communications difficult to decipher. However, as previously indicated, it is often nonetheless possible to monitor significant microphonic signals carried by the telephone lines when the telephones are on-hook.
Prior art devices are also known which inherently avoid microphonic behaviour (e.g. U.S. Pat. No. 4,091,232). Such devices, however, are of complex design and consequently fail to resemble contemporary office telecommunications devices. Another major drawback is that these devices are very expensive to manufacture and are therefore suited only for the highest security levels, where cost is not an object.
Therefore, it is an object of an aspect of this invention is to address the on-hook microphonic behaviour of telecommunications devices in a relatively simple cost-effective manner.